In 2002, the mobile phone service of the 3G system called the third generation in Japan started. Initially, small-size packets were transmitted and received for transmission of voice calls and mails. However, with the introduction of high speed downlink packet access (HSDPA), it became possible to transmit and receive larger-size packets for downloading of music files or streaming of video. With such an increase of packet capacities, the service of long term evolution (LTE) in which orthogonal frequency division multiple access (OFDMA) is used for downlink also started for the expansion on the side of the wireless network. In addition, the start of the 4G service is planned for around 2015. Accordingly, up to 1 Gbps (bit per second) is realized in a semi-fixed state and also up to 100 Mbps is realized even in a moving environment.
Also, for example, utilization of small cells is studied to handle hot spots at which traffic locally concentrates or to improve the utilization efficiency of frequency resource. Also, for example, there is an ongoing study on introduction of a frequency sharing technology for sharing frequency band, called white space, which is unused temporally or locally, between systems in accordance with a rule. Also, as disclosed in NPL1, in North America, the FCC is studying a scheme of frequency sharing for sharing frequency band that is unused temporally or locally, between the systems in accordance with a rule.